1. Field of the Invention
The present invention relates, in general, to pulverized coal burners used in a furnace, and in particular, to a new and useful arrangement for controlling the flow of combined pulverized coal and primary air through the burner nozzle, in a manner which results in low emissions of nitrogen oxides (NO.sub.x) and improved combustion efficiency.
2. Description of the Related Art
Pulverized coal-fired burners produce NO.sub.x emissions as a consequence of oxidation of fuel bound nitrogen found in coal (fuel NO.sub.x), oxidation of nitrogen in the combustion air at elevated flame temperatures (thermal NO.sub.x), and intermediate reactions of hydrocarbons with atmospheric nitrogen (prompt NO.sub.x). The largest source of emissions is from fuel NO.sub.x. These emissions can be reduced by altering the combustion process. Fuel nitrogen is released during the combustion of pulverized coal, during devolatilization and during char burnout. Fuel NO.sub.x is readily formed during devolatilization unless countermeasures are used, due to the high temperatures and abundance of oxygen available in the flame root. Fuel NO.sub.x formation tends to be less during char burnout since flame temperatures tend to decrease and oxygen partial pressure is reduced.
Conventional burners produce high levels of NO.sub.x. These burners allow rapid complete mixing of the coal and combustion air, favoring NO.sub.x formation. Some of the most effective pulverized coal-fired burners for reducing NO.sub.x emissions are designed to control and limit oxygen availability during devolatilization. In essence the fuel is separated from the majority of the combustion air during this brief period, and then introduced downstream in the flame. Two examples of such burners are disclosed in U.S. Pat. Nos. 3,788,796 and 4,836,772. These burners typically reduce NO.sub.x emissions by 40 to 60% below uncontrolled levels.
Known low NO.sub.x burners, however, tend to have elongated flames due to the axial injection of the fuel jet and staged introduction of combustion air. This can result in flame impingement on furnace walls opposite the burner location, and associated problems with slagging and or furnace tube corrosion. In addition, low NO.sub.x burners tend to produce higher levels of unburned carbon due to delayed and/or incomplete mixing of air and fuel. Unburned carbon directly contributes to a reduction in boiler efficiency and increased operating costs. Elevated levels of unburned carbon can also substantially increase ash disposal costs.
In order to reduce these tendencies, some low NO.sub.x burner designs employ mixing devices at or near the exit of the burner nozzle. These devices, many of which are referred to as impellers, deflect the coal outwardly away from the burner centerline. These devices may impart radial deflection, swirl, or some combination thereof, on the flame.
A patent which discloses a flow diverting mechanism at the end of a burner nozzle, is U.S. Pat. No. 2,380,463, which uses conically diverting vanes for diverting the mixture of primary gas and pulverized coal outwardly away from the axis of the nozzle.
U.S. Pat. No. 4,348,170 discloses the use of a tapered tip in a burner for confining the flame used in conjunction with diverters for dividing secondary combustion air around the flame. However, there is no use of crossing streams of primary air plus pulverized coal immediately downstream of the burner nozzle.
U.S. Pat. No. 4,397,295 teaches directing air radially inwardly with a slight tangential component, toward a flame to shape and shorten the flame. A distinctive swirling and violent mixing effect would be achieved. A similar mixing effect would be achieved with the invention of U.S. Pat. No. 4,523,529.
U.S. Pat. No. 5,347,937 discloses dividing a fuel/air mixture into multiple streams which do not rapidly mix with each other for the purpose of enhancing the conditions that reduce NO.sub.x formation.